Applications of Remote Sensing Techniques in
Mineral Exploration

Introduction

Remote Sensing is the science and art of acquiring information (spectral, spatial, temporal) about material objects, area, or phenomenon, without coming into physical contact with the objects, or area, or phenomenon under investigation. Without direct contact, some means of transferring information through space must be utilized. In remote sensing, information transfer is accomplished by use of electromagnetic radiation (EMR). EMR is a form of energy that reveals its presence by the observable effects it produces when it strikes the matter. The electromagnetic spectrum is the continuum of en­ergy that ranges from meters (radio waves) to fractions of nanometers (gamma rays) in wave­length, travels at the speed of light, and propagates through a vacuum such as outer space.  1 nanometer (nm) = 10-9 m.

When electro-magnetic energy is incident on any given earth surface feature, three fundamental energy interactions with the feature are possible – it is either reflected, absorbed or transmitted.  The proportions of energy reflected, absorbed, and transmitted will vary for different earth features, depending upon their material type and conditions. These differences permit us to distinguish different features on an image.  Even within a given feature type, the proportion of reflected, absorbed, and transmitted energy will vary at different wavelengths. Thus, two features may be distinguishable in one spectral range and be very different on another wavelength brand. Within the visible portion of the spectrum, these spectral variations result in the visual effect called COLOUR. For example we call blue objects 'blue' when they reflect highly in the 'green' spectral region, and so on. Thus the eye uses spectral variations in the magnitude of reflected energy to discriminate between various objects.  A graph of the spectral reflectance of an object as a function of wavelength is called a spectral reflectance curve. The configuration of spectral reflectance curves provides information about the identity of an object, and if it has a high resolution, it is unique to an object. Resolution of remotely sensed imageries is of different types:

Spectral resolution refers to the width or range of each spectral band being recorded. As an example, panchromatic imagery (sensing a broad range of all visible wavelengths) will not be as sensitive to vegetation stress as a narrow band in the red wavelengths, where chlorophyll strongly absorbs electromagnetic energy.

Spatial resolution refers to the discernible detail in the image. Detailed mapping of wetlands requires far finer spatial resolution than does the regional mapping of physiographic areas.

Temporal resolution refers to the time interval between images. There are applications requiring data repeatedly and often, such as oil spill, forest fire, and sea ice motion monitoring. Some applications like geological mapping need imaging only once.

Types

1.      In respect to the type of Energy Sources:

·        Passive Remote Sensing: Makes use of sensors that detect the reflected or emitted electro-magnetic radiation from natural sources.

·        Active remote Sensing: Makes use of sensors that detect reflected responses from objects that are irradiated from artificially generated energy sources, such as radar.

2.      In respect to Wavelength Regions:

·        Remote Sensing is classified into three types in respect to the wavelength regions

o     Visible and Reflective Infrared Remote Sensing. (0.38-3.0 nm)

o     Thermal Infrared Remote Sensing. (7.0-15.0 nm)

o     Microwave Remote Sensing. (1.0-1000 mm)

Each band of information collected from a remote sensing sensor contains important and unique data. We know that different wavelengths of incident energy are affected differently by each target - they are absorbed, reflected or transmitted in different proportions. In many applications, using information from different bands ensures that target identification or information extraction becomes fairly accurate.

Why remote sensing?

Remote sensing gives the overview required to 1) construct regional unit maps, useful for small scale analyses, and planning field traverses to sample and verify various units for detailed mapping; and 2) understand the spatial distribution and surface relationships between the units. VIR remote sensing provides the multispectral information relating to the composition of the unit, while radar can contribute textural information. Multiple data sources can also be integrated to provide a comprehensive view of the lithostratigraphy.

Stereo imagery can also facilitate delineation and identification of units by providing a three dimensional view of the local relief. Some rocks are resistant to erosion, whereas others erode easily. Identification elements such as weathering manifestations may be apparent on high or medium resolution imagery and airphotos.  Images or airphotos can be taken into the field and used as basemaps for field analysis.  Two different scales of mapping require slightly different imaging sources and parameters.

For site specific analysis, airphotos provide a high resolution product that can provide information on differential weathering, tone, and microdrainage. Photos may be easily viewed in stereo to assess relief characteristics.

Regional overviews require large coverage area and moderate resolution. An excellent data source for regional applications is a synergistic combination of radar and optical images to highlight terrain and textural information.

In either case, frequency of imaging is not an issue since in many cases the geological features of interest remain relatively static. Immediate turnaround is also not critical.

Requirements for remote sensing application do not differ significantly around the world. One of the biggest problems faced by both temperate and tropical countries is that dense forest covers much of the landscape. In these areas, geologists can use remote sensing to infer underlying lithology by the condition of vegetation growing above it. This concept is called "geobotany". The underlying principle is that the mineral and sedimentary constituents of the bedrock may control or influence the condition of vegetation growing above.

In reality, the topography, structure, surficial materials, and vegetation combine to facilitate geologic unit interpretation and mapping. Optimal use of remote sensing data therefore, is one that integrates different sources of image data, such as optical and radar, at a scale appropriate to the study.

Applications in Geological Mapping

We have seen that geological mapping provides the ground for any mineral exploration programme.  It involves the study of landforms, structures, and the subsurface, to understand physical processes creating and modifying the earth's crust. Geological mapping is absolutely essential for the exploration and exploitation of mineral, hydrocarbon and other energy resources. Remote sensing is used as a tool to extract information about the land surface structure, composition or subsurface.  Combined with data from other sources, it provides complementary measurements. Multispectral reflectance data can provide information on lithology, rock composition or rock alteration, which is so often associated with, and indicative of the presence of mineral deposits, particularly epigenetic deposits.   Radar provides an expression of surface topography and roughness, and thus is extremely valuable, especially when integrated with other data sources in providing details of relief or physiography.  Physiographic features, as we all know are excellent guides to the presence of ore.

Remote sensing is not limited to direct geology applications - it is also used to support logistics, such as route planning for access into a mining area, reclamation monitoring, and generating basemaps upon which geological data can be referenced or superimposed.

Geological applications of remote sensing include the following:

·        surficial deposit / bedrock mapping

·        lithological mapping

·        structural mapping

·        sand and gravel (aggregate) exploration/ exploitation

·        mineral exploration

·        hydrocarbon exploration

·        environmental geology

·        geobotany

·        baseline infrastructure

·        sedimentation mapping and monitoring

·        event mapping and monitoring

·        geo-hazard mapping

·        planetary mapping


Applications in Structural Mapping

Structural geology plays an important role in mineral and hydrocarbon exploration, and potential hazard identification and monitoring.

Structural mapping is the identification and characterization of structural expression. Structures include faults, folds, synclines and anticlines and lineaments. Understanding structures is the key to interpreting crustal movements that have shaped the present terrain. Structures can indicate potential locations of oil and gas reserves by characterizing both the underlying subsurface geometry of rock units and the amount of crustal deformation and stress experienced in a certain locale. Detailed examination of structure can be obtained by geophysical techniques such as seismic surveying.

Structures are also examined for clues to crustal movement and potential hazards, such as earthquakes, landslides, and volcanic activity. Identification of fault lines can facilitate land use planning by limiting construction over potentially dangerous zones of seismic activity.

The main advantage of remotely sensed data in structural mapping are that they provide some information on the spatial distribution and surficial relief of the structural elements. Radar is well suited to these requirements with its side-looking configuration. Imaging with shallow incidence angles enhances surficial relief and structure. Shadows can be used to help define the structure height and shape, and thus increasing the shadow effect, while shallow incidence angles may benefit structural analysis.

Certain remote sensing devices offer unique information regarding structures, such as in the relief expression offered by radar sensors. Comparing surface expression to other geological information may also allow patterns of association to be recognized. For instance, a rock unit may be characterized by a particular radar texture which may also correlate with a high magnetic intensity or geochemical anomaly. Remote sensing is most useful in combination, or in synergy, with complementary datasets.

In areas where vegetation cover is dense, it is very difficult to detect structural features. A heavy canopy will visually blanket the underlying formation, limiting the use of optical sensors for this application. Radar however, is sensitive enough to topographic variation that it is able to discern the structural expression reflected or mirrored in the tree top canopy, and therefore the structure may be clearly defined on the radar imagery.

Structural analyses are conducted on regional scales, to provide a comprehensive look at the extent of faults, lineaments and other structural features with which ore deposits are frequently associated. Aerial photos can be used in temperate areas where large-scale imagery is required, particularly to map relief which is quite often determined by structure.

The structural information provided by radar complements other spatial datasets. When integrated together, SAR and spatial geological datasets provide a valuable source of geological information

Applications in Lithological Mapping

Mapping geologic units consists primarily of identifying physiographic units and determining the rock lithology or coarse stratigraphy of exposed units. These units or formations are generally described by their age, lithology and thickness. Remote sensing can be used to describe lithology by the colour, weathering and erosion characteristics (whether the rock is resistant or recessive), drainage patterns, and thickness of bedding.

Unit mapping is useful in oil and mineral exploration, since these resources are often associated with specific lithologies. Structures below the ground, which may be conducive to trapping oil or hosting specific minerals, often manifest themselves on the Earth's surface. By delineating the structures and identifying the associated lithologies, geologists can identify locations that would most likely contain these resources, and target them for exploration.

In terms of remote sensing, these "lithostratigraphic" units can be delineated by their spectral reflectance signatures, by the structure of the bedding planes, and by surface morphology.

Applications in Hydrology

Hydrology is the study of water on the Earth's surface, whether flowing above ground, frozen in ice or snow, or retained by soil. Hydrology is inherently related to many other applications of remote sensing, particularly forestry, agriculture and land cover, since water is a vital component in each of these disciplines. Most hydrological processes are dynamic, not only between years, but also within and between seasons, and therefore require frequent observations. Remote sensing offers a synoptic view of the spatial distribution and dynamics of hydrological phenomena, often unattainable by traditional ground surveys. Radar has brought a new dimension to hydrological studies with its active sensing capabilities, allowing the time window of image acquisition to include inclement weather conditions or seasonal or diurnal darkness.


Examples of hydrological applications include:

·        wetlands mapping and monitoring,

·        soil moisture estimation,

·        snow pack monitoring / delineation of extent,

·        measuring snow thickness,

·        determining snow-water equivalent,

·        river and lake ice monitoring,

·        flood mapping and monitoring,

·        glacier dynamics monitoring (surges, ablation)

·        river /delta change detection

·        drainage basin mapping and watershed modelling

·        irrigation canal leakage detection

·        irrigation scheduling

 

Article compiled from information provided through the website of Canada Centre for Remote Sensing at http://www.ccrs.nrcan.gc.ca/ccrs/homepg.pl?e

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